1. Field of the Invention
The present invention relates to a device and a method for seat supervision in motor vehicles and in particular to a device vice and a method for optical seat supervision which are suitable able for controlling the activation state of one or more airbags.
2. Description of Prior Art
A centrally situated airbag should fire in the event of an accident if the seat is occupied by a person in the upright position. If a person bends forward too close to the airbag or if there is a child""s seat or some other object on the seat, so that a person or a child could be injured or the airbag be triggered unnecessarily, this situation must be detected by a seat supervision system so as to prevent the airbag from firing or to modify the unfolding dynamics of the airbag.
The advantages of an airbag far outweigh potential disadvantages. However, an airbag can only provide protection with full efficacy if the occupant of the motor vehicle is belted up, so the airbag must be regarded as a supplementary unit of the so-called restrain system, SRS (SRS =Supplemental Restrain System). The airbag supports the restraining action of the belt in the event of a frontal collision and prevents direct impact of the thorax, neck and head with the dashboard.
An airbag normally fires 10 to 40 ms after the collision has occurred if the change in speed resulting from the collision exceeds a specified value, 25 km/h in Europe and 15 km/h in the USA. The airbag envelope then emerges from the airbag cover at a speed of 250 to 300 km/h. The time taken to inflate the airbag is 25 to 40 ms, which is faster than the eyelid reaction time. Only after this inflation time does the airbag provide its full protection. If, at the moment an accident occurs, an occupant is not in the normal sitting position, a situation which is referred to as OOP (Out of Position), or if there is a child""s seat on the front passenger seat, the high acceleration rates during the unfolding phase of the airbag, which can be of the order of 100 G, can result in serious injuries, independently of whether the change of speed when the airbag is fired is low or high. Thus it is possible that the consequences of an accident which would not be that serious may be made worse. The airbag can only exhibit its full efficacy when the occupant is situated at a sufficient distance from the airbag cover. In the normal sitting position the distance between the driver""s chest and the hub of the steering wheel is usually about 30 cm, whereas the distance between the front passenger""s chest and the airbag cover is usually about 50 cm, the normal sitting positions varying according to the interior configuration of the vehicle.
In the field of technology dealt with here a plurality of methods for seat supervision in motor vehicles is already known by means of which airbag activation state control can be effected on the basis of seat supervision.
First of all, it is possible to deactivate and activate the airbag manually so as to avoid the chance of being exposed to danger by the airbag in certain special cases. However, with this approach it is easy to forget the need to activate the airbag. Using a pressure sensor arrangement to measure the weight exerted on a seat of the motor vehicle is a method which is often used to detect seat occupancy, the activation/deactivation of the airbag being effected on the basis of the measured weight. With this method, however, children""s seats which weigh less than the weight threshold employed are not detected. Furthermore it is possible for the airbag to be activated unnecessarily if there is a load on the front passenger seat.
It is also possible to detect seat occupancy by performing a capacitive measurement of the object, usually a dielectric, which is situated between the seat and the instrument panel. For this purpose electrodes are located in the seat and in the instrument panel. With the aid of the permittivity measurement it is possible to detect the occupancy state of a seat, though an error-free distinction between a human being and an object will not generally be possible.
Also known is the practice of employing propagation time measurement methods using active signal sources, e.g. radar signal sources or laser light sources, to survey seat surfaces to obtain a xe2x80x9cprofilexe2x80x9d for the occupancy evaluation.
Methods for children""s seat detection are also known which are based on the use of special children""s seats equipped with transponder technology. These transponders in the children""s seats deactivate the airbag. With these methods based on transponder technology it is not, however, possible to detect either foreign children""s seats or vehicle occupants.
From the article xe2x80x9cFuture Applications of Microsystem Technologies in Automotive Safety Systemsxe2x80x9d by P.Steiner and S.Schwehr, in Ricken/Gessner: Advanced Microsystems for Automotive Applications 1998, Springer Verlag Berlin, 1998, a passenger state detection method is known in which a person endangering zone immediately in front of an airbag is defined, this zone being supervised at various points by means of active monochromatic light sources (LEDs) and traditional CCD image sensors. When an occupant of the motor vehicle abandons the normal position, which is situated at a typical distance from an airbag module, and thus possibly comes too close to the airbag, the system described in the above article deactivates the airbag or reduces the firing power of the airbag. According to this publication the evaluation of the occupancy detection follows the triangulation principle, whereby the location of an object can be established from the position of a light spot in the image plane of a camera. The triangulation principle is known in this area of technology.
Another alternative method for selectively controlling airbag activation and deactivation is to define a person endangering zone immediately in front of the airbag and to supervise this zone with the aid of a light curtain or a light barrier. If the light curtain or the light barrier is interrupted, indicating that someone is in the danger zone, the airbag is deactivated.
The major problem with the above-cited optical methods for seat supervision using active light sources arises because of the extreme differences in illumination which can occur in a motor vehicle. The illumination can vary between 1 mlx on a dark night and 100 klx in direct sunlight. Especially in the case of simultaneous direct sunlight, the light beams emitted into a supervised space by active monochromatic sources can no longer be resolved. Furthermore, as a consequence of the statuary, country-specific safety regulations for guaranteeing the radiation safety of laser beams, the monochromatic light source may only emit a small amount of light energy.
It is the object of the present invention to provide a device and a method for seat supervision in motor vehicles which permit rapid detection of seat occupancy as a means of activating and deactivating an airbag under all illumination conditions.
According to a first aspect of the invention, this object is achieved by a device for seat supervision in motor vehicles which features a pulsed monochromatic light source for the short-time illumination of a space to be supervised. In addition an image sensor array for recording an image of the supervised space is provided which has a global electronic shutter unit which enables the illumination time of the image sensor array to be set independently of the readout clock of the image sensor array. Also provided is a synchronization unit for synchronizing the operation of the pulsed laser source and of the image sensor array. The device according to the present invention also has a control unit for controlling the light source and the image sensor array so as to create a first image of the supervised space during illumination of the same at a first time and to create a second image of the supervised space without illumination of the same at a second time. Finally, a unit is also provided to create a difference signal from the first and the second image.
The device according to the present invention is suited, on the one hand, to the use of the optical light section principle so as to determine the contour of an object on a seat under supervision and thus to identify the object on the seat. On the other hand the device according to the present invention is suited to determining the distance between a person or an object and an airbag, so that if a person suddenly makes a movement towards the airbag this can be deactivated if it is approached too closely. The preferred method here is to use a multizone investigation which permits power-adapted firing of the airbag.
The image sensor array of the device for seat supervision according to the present invention is preferably a CMOS/CCD image sensor with global shutter function which is affixed together with a monochromatic illumination source at a suitable location in the motor vehicle, e.g. in the rooflining, on the instrument panel or in the cowl. Global shutter function means that a shutter unit is provided which permits all the sensor elements of the sensor array to be shuttered electronically at the same time.
According to the present invention a scene is evaluated using difference image signals resulting from an image recorded with very short-time pulsed active illumination and an image recorded without pulsed illumination. The pulsed and the unpulsed image, i.e. the image without illumination by the light source, are recorded consecutively with minimal time lapse. The device according to the present invention thus operates substantially trouble-free even under extreme ambient conditions. Preferably an interference filter located in front of the image sensor array and which is tuned to the wavelength of the light beam emitted by the monochromatic illumination source can also be used so that a considerable part of the interfering light can also be suppressed.
The resolvable intensity differences in the difference image created in the present invention can subsequently be evaluated using conventional analysis methods employed in digital image processing and pattern recognition. Experience gained in the area of industrial measurement technology with light section systems under defined illumination conditions is particularly useful here. Triangulation methods are preferably employed to determine the distance of an occupant, if identified as such, from the airbag, the activation state of the airbag then being controlled on the basis of this distance determination.
According to a second aspect of the present invention, the above object is achieved by a method for seat supervision in motor vehicles wherein a supervised space is first illuminated by a pulsed monochromatic light source employing an illumination time in the microsecond or nanosecond range. During this illumination a first image of the supervised space is recorded by means of an image sensor array. A second image of the supervised space without illumination of the same by the light source is recorded a maximum of 10 ms before or after recording the first image. A difference image is then created from the first and second images.
The present invention is based on the finding that trouble-free seat supervision in motor vehicles can be achieved by using devices and methods which create a difference image from a first image recorded during an illumination and a second image recorded without illumination. For the creation of such a trouble-free difference image signal suitable for signal evaluation certain conditions must be met. First, short-time light pulses in the microsecond or nanosecond range must be used for the illumination so that the highest possible light power can be delivered with minimal light energy. In addition the time intervals between the images recorded with pulsed and unpulsed light must be as short as possible so as to minimize disturbances caused by object movement. Furthermore, globally synchronized illumination of the image sensor is essential in order to minimize movement fuzziness and the falsifications in the difference image signal which result therefrom.
In the device according to the present invention the above conditions can be realized through the use of an image sensor array with a global electronic shutter unit and a synchronization unit for synchronizing the operation of the pulsed light source and the image sensor array.
In known systems in the field of optical supervision technology, image recording methods using a continuous, active, structured illumination are employed. This makes it possible to achieve defined illumination conditions, a precondition for robust image evaluation. This approach is followed in multifarious areas of industrial image processing for shape testing and detection. In contrast, the present invention employs pulsed light sources with very short illumination times in combination with an image sensor array which permits sufficiently short global illumination times and which is synchronized with the active illumination source. This first makes it possible, even under extreme illumination conditions, to radiate the structured light emitted by the light source with an energy which permits the light reflected from an object to be detected despite the effect of sunlight without the injuries which may be caused by the light from laser sources or focused LEDs impinging directly on the eye. The combination of pulsed light sources with structured illumination and the use of an image sensor array, preferably a CMOS/CCD image sensor, with global shutter unit and variable integration times in the microsecond and nanosecond range, constitutes a particularly advantageous system for seat supervision which is e.g. suitable for controlling the activation state of an airbag.
By creating a difference image the present invention thus makes it possible to eliminate extraneous light effects in the form of direct sunlight, shadows, frontal light, street lighting and so on. Only the invisible, narrow-band near-infrared region of the light source contributes to the signal to be evaluated.
Further developments of the present invention are described in the dependent claims.